There is a great need in the semiconductor industry for metrology equipment that can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. In recent years, a number of products have been developed for the nondestructive evaluation of semiconductor samples. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,646,088; 5,854,710; 5,074,669 and 5,978,074. Each of these patents is incorporated herein by reference.
In the basic device described in the patents, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample that spread out from the pump beam spot. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.
The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. As a result, subsurface features that alter the passage of the thermal and plasma waves have a direct effect on the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.
In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused collinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of reflected probe beam. The photodetector generates an output signal that is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface. The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation frequency. A lock-in detector is typically used to measure both the in-phase (I) and quadrature (Q) components of the detector output. The two channels of the output signal, namely the amplitude A2=I2+Q2 and phase Θ=arctan (I/Q) are conventionally referred to as the photomodulated reflectivity (PMR) or Thermal Wave (TW) signal amplitude and phase, respectively.
Dynamics of the thermal- and carrier plasma-related components of the total PMR signal in a semiconductor is given by the following general equation:
            Δ      ⁢                          ⁢      R        R    =            1      R        ⁢          (                                                  ∂              R                                      ∂              T                                ⁢          Δ          ⁢                                          ⁢                      T            0                          +                                            ∂              R                                      ∂              N                                ⁢          Δ          ⁢                                          ⁢                      N            0                              )      
where ΔT0 and ΔN0 are the temperature and the carrier plasma density at the surface of a semiconductor, R is the reflectance, dR/dT is the temperature reflectance coefficient and dR/dN is the carrier reflectance coefficient. For silicon, dR/dT is positive in the visible and near-UV part of the spectrum while dR/dN remains negative throughout the entire spectrum region of interest. The difference in sign results in destructive interference between the thermal and plasma waves and decreases the total PMR signal. The magnitude of this effect depends on the nature of a semiconductor sample and on the parameters of the photothermal system, especially on the pump and probe beam wavelengths.
In the early commercial embodiments of the TP device, both the pump and probe laser beams were generated by gas discharge lasers. Specifically, an argon-ion laser emitting a wavelength of 488 nm was used as a pump source. A helium-neon laser operating at 633 nm was used as a source of the probe beam. Lore recently, solid state laser diodes have been used and are generally more reliable and have a longer lifetime than gas discharge lasers. In the current commercial embodiment, the pump laser operates at 780 nm while the probe laser operates at 670 nm.
In practice, the use of the visible spectrum for both pump and probe beams has proven to be effective for a broad range of practical applications. Alternatively, U.S. Pat. Ser. No. 5,034,611 discloses a PMR system having a 488 nm pump beam and a beam probe in the UV range of 200 through 345 nm. That particular combination is believed to be an effective tool for measuring implantation doses above 1015 cm−2 at relatively shallow depth (i.e., approximately 10 nm).
As may be appreciated, it is entirely possible to construct PMR systems that operate at probe and pump wavelengths that differ from the systems described above. As will be described below, there are applications that benefit from these alternate configurations. This is particularly true for applications that involve relatively high temperature reflectance coefficients.